how do i change my wifi band from 2.4GHz to 5GHz in my Macbook air

The Wi-Fi card in your MacBook Air can connect to either 2.4 or 5 GHz wireless networks. There is no switch to disable which radio it can connect to. If the Wi-Fi router support simultaneous dual-band operation, and each band has a different wireless network name or BSSID, you an "force" your MBA to connect to either network by just choosing it from the Wi-Fi menu icon.

Hi,

Go up to the Wi-Fi icon in the top menu bar and select the network on which your router is broadcasting 5GHz.

Change the frequency your Router broadcasts from 2.4GHZ to 5GHZ.